1. Field of the Invention
This invention relates to a process for improving the productivity of catalysts used in processes for the polymerization of ethylene and the copolymerization of ethylene with alpha-olefins.
2. Prior Art
U.S. Pat. No. 4,302,565 to Goeke et al. describes high activity catalysts prepared by forming a precursor from at least one titanium compound, at least one magnesium compound and at least one electron donor compound; impregnating said precursor on a support; and activating said impregnated precursor with an organoaluminum compound. Other U.S. Pat. Nos. such as 4,395,359, 4,379,759, 4,354,009, 4,293,673, 4,376,062, 4,303,771, 4,383,095 and 4,379,758 also teach catalyst precursors formed by combining magnesium compounds, titanium compounds and electron donor compounds followed by precursor activation.
The prior art indicates that heating such catalyst precursors above a certain temperature range has a negative effect on catalyst productivity. U.S. Pat. No. 4,427,573 to Miles et al., for example, teaches at page 17 (Table III) that heating at temperatures over 120.degree. C would have a deleterious effect on productivity.
Surprisingly, it has now been found that heating Ti(III) based precursors within a narrow elevated temperature range to remove a specified amount of the electron donor significantly increases catalytic activity.